add support for list of AP connections

- auto select ssid with best signal
 - for debugging enable DEBUG_WIFI_MULTI macro and call Serial.setDebugOutput(true);

change ESP8266WiFiClass::status() return type to wl_status_t

libraries/ESP8266WiFi/src/ESP8266WiFi.h

@@ -195,7 +195,7 @@ public:
      *
      * return: one of the value defined in wl_status_t
      */
-    uint8_t status();
+    wl_status_t status();
 
     /*
      * Resolve the given hostname to an IP address.